In the process of fixing the ceilingfrom a leak upstairs we discovered a HUGE return unfinished air vent that was open and  just sucking in air from between the rafters.  We moved in the house new so I know it wasn't done after the fact.  This also explains why the house may be so dusty all the time and has never cooled off properly.  At any rate can anyone think of a legitimate reason for them doing this??

Thanks